1. Field
This invention relates generally to radio wave antennas and more particularly to a balanced doublet dipole antenna.
2. Related Art
An RFID system includes an RFID tag and an RFID reader. The reader includes a radio frequency (RF) transmitter and an RF receiver. The tag includes a microchip and an antenna. One type of tag is a passive tag which lacks its own power supply. One type of passive tag receives far-field EM waves transmitted by a reader. Typically, the far-field EM waves are modulated using a type of amplitude shift keying (ASK) to convey power to this type of passive tag via the EM waves. The antenna of the passive tag is tuned to a particular frequency, and the antenna of the passive tag is designed such that it has a preselected impedance. Terminals of the antenna of the passive tag are coupled to input terminals of the microchip of the passive tag.
If the preselected impedance of the antenna matches an input impedance of the input terminals of the microchip, the antenna absorbs much of the energy that reaches it at the particular frequency range. Such energy appears across the input terminals of the microchip. A diode in the microchip rectifies the AC voltage, and the rectified voltage is stored in a capacitor of the microchip. The microchip uses the stored rectified voltage to power itself, and, more specifically, to change its input impedance over time. The microchip changes its input impedance between two states: one state is closely matched to the preselected impedance of the antenna and another state is greatly mismatched to the preselected impedance of the antenna. An input impedance of a microchip and an impedance of an antenna are closely matched when they are complex conjugates of each other.
When the impedance of the microchip is greatly mismatch to the impedance of the antenna, most of the EM waves that reaches the antenna at the particular frequency is reflected from the antenna. Such reflection is called back scattering. As a result of the microchip changing its input impedance, such back scattering of the EM waves is modulated. The modulation encodes at least identification of the passive tag. During an interval that the reader is not transmitting, some of the EM waves that the passive tag back scatters are received by the reader, and the reader receives the identification of the passive tag. The reader may be coupled to a computer that processes the identification of the passive tag.
Important performance characteristics of an RFID antenna are impedance, bandwidth and gain. Important performance characteristics of an RFID tag are sensitivity, turn-on power, and return loss. An important performance characteristic of an RFID system is reading range which is a maximum distance at which a reader can either read information from, or write information to, a tag. Reading range is determined by a rate of successful reads or by a rate of successful writes, which varies with distance and which depends on characteristics of the reader, characteristics of the tag, and on a propagation environment.
RFID technology includes a low frequency band, a high frequency band, an ultra-high frequency (UHF) band, and a microwave band. Each International Telecommunication Union region has its own specific frequency allocation within the UHF band. For example, Europe (Region 1) allocates 866-869 MHz, North and South America (Region 2) allocates 902-928 MHz, and Asia (Region 3) allocates 950-956 MHz. The frequency range 902-928 MHz is also referred to as an industrial, scientific and medical (ISM) radio band. It should be noted that the central frequency of this ISM band is 915 MHz.